1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image on an original document to acquire image data representing the image of the original document, an image forming apparatus for forming the image of the original document as a duplicate and an image reading method for reading the image on the original document.
2. Description of the Related Art
Intensification of resolution and accompanying improvement of productivity have been demanded in an image reading apparatus such as a scanner and a copier. Increasing of processing speed by loading with a high-frequency drive clock has been widely conducted.
An influence upon surrounding apparatuses of the high frequency radiated noise originating from a high-frequency clock has been discussed recently and there is a possibility that a fatal accident may occur. Thus, legal restrictions demanding suppression of radiated noise have been becoming strict. As a countermeasure for the radiated noise, there has been well known a frequency dispersion technology which reduces the peak level of the radiated noise by modulating the oscillation frequency of the drive clock continuously at a predetermined period.
FIG. 1 is a diagram for explaining the frequency dispersion technology.
FIG. 1(A) shows an example of a modulation pattern for dispersing the frequency of a reference clock. A modulation pattern A changes the frequency of clock in DOWN direction (direction that increases the period) and UP direction (direction that decreases the period) continuously. A reference clock A_1 having a predetermined frequency is modulated to a frequency dispersion clock A_2 in which the frequency is changed continuously at a modulation period T by this modulation pattern A and this frequency dispersion clock A_2 is used as a drive clock instead of the reference clock A_1. The modulation period signal A_3 is a signal outputted at a timing synchronous with the modulation period T of the frequency dispersion clock A_2.
FIG. 1(B) shows radiated noise levels when the reference clock A_1 and the frequency dispersion clock A_2 shown in FIG. 1(A) are used as each drive clock. A radiated noise level B_1 corresponding to the reference clock A_1 has a peak at a frequency Fp while in the radiated noise level B_2 of the frequency dispersion clock A_2, the frequency in the vicinity of the peak is dispersed and the peak level drops by that amount. In this way, the peak level of the radiated noise can be reduced by using the frequency dispersion clock A_2 as a drive clock.
Here, an ordinary example in which the above-described frequency dispersion technology is applied to a line reading type image reading apparatus which reads an image of an original document by scanning will be described.
The line scanning type image reading apparatus scans a scanning object line in a main scanning direction synchronously with the drive clock with an image pickup device, for example, a charge couple device (CCD) by moving the scanning object line in an auxiliary scanning direction in each predetermined line period. In the meantime, the image pickup device used generally is an analog device which stores electric charges by receiving light according to the phase of the drive clock or pulse width so as to output an analog signal. Therefore, if the frequency dispersion clock A_2 having a varied phase or pulse width is adopted as the drive clock, the signal strength of an outputted analog signal is deflected depending on a change in the frequency dispersion clock A_2, so that noise is added to image data generated based on the analog signal. Further, there occurs such a problem that beat noise which is oblique stripe is generated in an image represented by that image data because the deflected position differs depending on each line.
As a method for eliminating such a default, a method of correcting that beat noise to a vertical stripe which is easy to correct by synchronizing the scanning start timing of the line with the frequency dispersion clock of the drive clock has been proposed (for example, Japanese Patent Application Laid-Open No. 2001-77989, Japanese Patent Application Laid-Open No. 2001-281252, and Japanese Patent Application Laid-Open No. 2003-8845).
FIG. 2 is a diagram showing the relation between the modulation period and line scanning start timing based on the method described in the above-described patent documents.
As shown in FIG. 2, the output interval I100 of a line signal A_100 which indicates the scanning start for a line is integer times the modulation period T of the modulation period signal A_3 and rise-up timings t1, t3 of the line signal A_100 are synchronous with the rise-up timing of the modulation period signal A_3. In this case, an influence of the frequency dispersion clock is generated in each line and consequently, beat noise occurs at the same position of each line, so that the beat noise is corrected to a vertical stripe.
When an image is read by changing the magnification in an image reading apparatus, the image reading speed is changed corresponding to the magnification. For example, when an image is read at a reduction rate of 50%, the reading speed is twice that at the time of reading at an equal magnification (magnification is 100%) by reducing reading resolution or the like. Therefore, the line signal A_50 at the time of reading at 50% is outputted at an output interval half(I-50) that of the output interval I_100. At this time, the second rise-up timing t2 of the line signal A_50 does not match with the rise-up timing of the modulation period signal A_3 and thus, the deflection position of the frequency dispersion clock differs depending on each line. Consequently, the beat noise is deviated from the vertical stripe. According to the above-described method, the beat noise can be corrected to the vertical stripe at the time of reading at the equal magnification. However, the beat noise may not be corrected to the vertical stripe, depending on the magnification at the time of reading in enlargement or reduction. Consequently, there is such a problem that the beat noise cannot be corrected easily.